To build an optical signal distribution network within a semiconductor substrate, one needs to make good optical waveguides to distribute the optical signals, and one needs to fabricate elements that convert the optical signals to electrical signals in order to interface with other circuitry. Extracting the optical signals can be accomplished in two ways. Either the optical signal itself is extracted out of the waveguide and delivered to other circuitry that can convert it to the required form. Or the optical signal is converted into electrical form in the waveguide and the electrical signal is delivered to the other circuitry. Extracting the optical signal as an optical signal involves the use of mirrors, gratings or couplers within the waveguides, or other elements that function like these devices. The scientific literature has an increasing number of examples of technologies that can be used to construct such devices. Extracting the optical signal as an electrical signal involves the use of detectors within the waveguide, i.e., circuit elements that convert the optical signal to an electrical form. The scientific literature also has an increasing number of examples of detector designs that can be used to accomplish this.
The challenge in finding the combination of elements that produces an acceptable optical distribution network becomes greater, however, when one limits the frame of reference to particular optical signal distribution network designs and takes into account the practical reality that any such designs should be relatively easy to fabricate and financially economical.
The combination of silicon and SiGe alloys (e.g. SixGe1-x) has attracted attention as useful combination of materials from which one might be able to easily and economically fabricate optical signal distribution networks. With SiGe alloys it is possible to fabricate waveguides in the silicon substrates. The index of refraction of a SiGe alloy is slightly higher than that of silicon. For example, a SiGe alloy with 5% Ge (i.e., Si0.95Ge0.05) has an index of refraction of about 3.52 while crystalline silicon has an index of refraction that is less than that, e.g. about 3.50. So, if a SiGe alloy core is formed in a silicon substrate, the difference in the indices of refraction is sufficient to enable the SiGe alloy core to contain an optical signal through internal reflections. Moreover, this particular combination of materials lends itself to the use of conventional silicon based semiconductor fabrication technologies to fabricate the optical circuitry.
Of course, for such a system to work as an optical signal distribution network, the optical signal must have a wavelength to which both the Si and the SiGe alloy are transparent. Since the bandgap energy of these materials is approximately 1.1 eV, they appear transparent to optical wavelengths having a wavelength greater than 1150 nm. A further reduction in bandgap energy caused by use of a SiGe alloy rather than pure Silicon, and higher temperature operation as high as 125° C. may further require the wavelength be longer than 1200 nm or even 1250 nm for very low absorption loss (approximately 1 db/cm or less). But, the transparency of these materials to optical signals having those wavelengths brings with it another problem. These materials are generally not suitable for building detectors that can convert the optical signals to electrical form. To be a good detector, the materials must be able to absorb the light in a manner so as to create useful charge that can be detected electrically. That is, the optical signal must be capable of generating electron transitions from the valence band to the conduction band within the detector to produce an electrical output signal. But the wavelengths greater than 1150 nm are too long to produce useful absorption by electron transitions in silicon, or in Si0.95Ge0.05 alloys at room temperature. At a wavelength of 1300 nm, the corresponding photon energy is about 0.95 eV, well below the room temperature bandgap of silicon and Si0.95Ge0.05 and consequently well below the amount necessary to cause transitions from the valence band into the conductor band.